Although the concept that sensitivity to the contingency between behavior and their consequences and the adeptness of the acquisition of operant control over the environment is central to many theories of psychological development, there has been literally no research on infant learning which has approached the topic from the standpoint of individual differences. The proposal describes a paradigm for the assessment of the stability of individual differences on these aspects of learning across both short- and longer-term time periods. The proposed studies provide a good first step toward the investigation of individual differences in infant learning and their potential meaning for later developmental outcome. The application proposes to study two samples of infants. One sample will include 90 infants from three different age groups (3, 6, and 9 month-olds) who will be tested twice within a period of 2 weeks in order to determine the within-age test-retest reliability of traditional measures of learning. A second sample of 30 infants will be tested once each at 3, 6, and 9 months of age in order to establish the longer-term stability of these measures across periods of major behavioral change in the first year of life. A second set of analyses are also proposed on these data to test the relationships between measures of individual response acquisition, extinction, and attention to the reinforcement/contingency, and the effects of repeated conditioning-extinction cycles of subsequent conditioning performance. Normative analyses of these measures will also be available from the data, and the overlap of the two samples in the design allows for independent replication of the effects tested for throughout the study.